So what do ruby slippers have to do with writing a novel? Think of it as a plot device -- like Hitchcock's MacGuffin -- something that represents both the protagonist's and the antagonist's goals. It's the thing that they are each willing to do almost anything to obtain or protect or destroy.
In The Wizard of Oz, Dorothy wants to get home; she needs the ruby sipper (though she doesn't realize it) to get there. The with wants the magic powers that the ruby slippers contain, but she has to kill Dorothy to get them. Single object; competing goals
Each time I develop a plot, I try to find the story's "ruby slippers" -- a single object that either embodies (Dorothy's ruby slippers) or represents (Katniss Everdeen's mockingjay pin) what both the villain and the hero will risk their lives to obtain or protect. Sometimes it's easy to identify -- priceless diamond necklace, or an inheritance, or an incriminating photograph that both the hero and the villain want. Sometimes it's an object that symbolizes a goal, the way Katniss Everdeen's mockingjay pin represents freedom and the survival of her people.
In Never Tell a Lie, the "ruby slippers" are Ivy's unborn baby. In my new novel coming out in April, There Was an Old Woman, it's a house. I'm still trying to find the ruby slippers in the novel I have underway, but so far it looks like it's a ring. I'm thinking maybe the setting should have a few rubies in it.
Hallie Ephron tries to keep her readers up nights. Never Tell a Lie (starred PW review) was adapted for film. In There Was an Old Woman, a woman returns her childhood home, only to find a hoarder's nightmare; her elderly neighbor may hold the clue to how it got that way.